Solid electrolyte oxygen sensors are widely used for monitoring high temperature combustion systems in order to maximize combustion efficiency. The major components of these sensors include a stabilized zirconia electrolyte cell which is connected to electronic measuring equipment by long lengths of lead wires. The material currently used for these lead wires is almost exclusively platinum. It has therefore been a long sought after goal in the art of oxygen analyzers to minimize the use of platinum or other noble metal leads since such materials are relatively expensive and platinum itself is considered a strategic material to the United States.
In order to minimize the use of platinum in its oxygen probes, Westinghouse Electric Corporation, in its model 218 probe, grounds the platinum lead from one surface of the solid electrolyte cell surface to a stainless steel mounting bracket which supports the electrolyte cell. The stainless steel mounting bracket is in turn grounded through intermediate members to the furnace wall. With this mechanical configuration, the use of platinum is effectively reduced since only one, rather than two platinum leads are required to pass from the electrolyte cell to the electrical measurement circuitry which is also grounded to the furnace wall. The existence of various junctions between dissimilar metals in the ground path of this design, however, can generate thermoelectric differences in the output of solid electrolyte cell and electrical compensation is required.
In another effort to minimize the use of platinum wire and to allegedly facilitate on-site servicing of an oxygen probe, U.S. Pat. No. 4,284,487, assigned to the Milton Roy Company, teaches a probe design in which the platinum lead wires are connected by nickel alloy screws and crimps to nickel alloy rod terminals situated in the hot combustion zone. Since all of the aforementioned electrical connections are made at approximately the same temperature, the likelihood of unbalanced thermocouples is minimized. In order to remove the connectors for the servicing of the probe, the other ends of the nickel alloy rod terminals are formed into the prongs of a male-type plug connector that mates with a female-type plug for disassembly. While the use of the design taught by the aforesaid patent can reduce the use of platinum, there are several potential problems. At the typical cell operating temperature of 1550.degree. F. (843.degree. C.) these nickel alloy parts are susceptible to rapid oxidation and the oxide scales formed on the nickel alloy screw, crimp and terminals surfaces can act as insulating surfaces and render the several connectors electrically discontinuous. More particularly, such oxidation occurs at temperatures higher than about 1000.degree. F. (538.degree. C.). Moreover, the nickel-alloy-platinum interfaces as well as the nickel alloy parts in contact with each other can easily fuse together at the high operating temperature via solid-state diffusion and become nondetachable. For these reasons the application of the referenced design is primarily limited to temperatures below 1000.degree. F. (538.degree. C.). It is thus inadequate for many industrial applications where temperatures as high as 1600.degree. F. (868.degree. C.) are commonly encountered.
It is therefore an object of this invention to replace, in one embodiment of the invention, all of the platinum lead wires extending from the zirconia electrolyte cell, and in an alternative embodiment, to eliminate a substantial portion of the platinum lead wires.
It is a further object of the present invention to provide a design for lead wires in an oxygen probe that will provide the desired satisfactory operating characteristics of all the replaced platinum lead wires at the cell operating temperature of 1550.degree. F. (843.degree. C.).
It is yet another object of the present invention to lower the cost of such high temperature oxygen probes by eliminating all or substantially all of the platinum lead wires which are utilized therein.